1. Field of Invention
This invention is directed to semiconductor devices that have reduced thermal resistance.
2. Description of Related Art
Some semiconductor devices experience poor performance because the heat that is generated during operation is not able to flow out of the semiconductor device efficiently. This leads to an increase in temperature that is detrimental to the performance of the semiconductor device. In many cases the temperature rise (xcex94T) is proportional to the heat per unit time (xcex94W) that is generated by the semiconductor device, i.e., xcex94T=Rxcex94W. Here, the proportionality factor R is the thermal resistance of the semiconductor device.
The inefficient heat flow out of the semiconductor device is often attributable to those parts of the device that have low thermal conductivity. In some semiconductor devices, during operation, heat must flow from the point where the heat is generated to an external heat sink. The heat sink has a sufficiently large thermal mass so that its temperature remains equal to the ambient air. However, to reach the heat sink, the heat must often flow through a region of the device that has a low thermal conductivity. In this case, the thermal resistance R of the semiconductor device will be high. As a result, the temperature in the semiconductor device will be much higher than the temperature of the heat sink.
In light emitting diodes and lasers, heat is generated in the active region of the device and in the p- and n-contacts. This heat must usually flow through the substrate to reach the external heat sink. Light emitting diodes and lasers can be formed on substrates that have poor thermal conductivities. Sapphire, a commonly used substrate, has, for example, a thermal conductivity (Kth) of 0.42 W/cmK at room temperature.
The light output intensity of a light emitting device depends on the temperature at which the light emitting device operates. With a constant current flowing through such a light emitting device, the light output intensity is reduced as the temperature increases. In some cases, high temperatures will prevent lasing in laser diodes and the like. In semiconductor devices that have two or more light emitting devices adjacent to each other, the light ouput intensity of the first light emitting device is affected by the output power of the adjacent light emitting devices. This occurs because the temperature in the first device is affected by the amount of dissipated power, and therefore the amount of heat, that is generated by the adjacent devices. This effect is known as thermal cross-talk. For many applications, e.g., laser printing, cross-talk between adjacent light emitting devices is highly undesirable, because the light emitting devices are desirably separately addressable and completely independent from each other.
Calculations of the temperature distribution in semiconductor devices have shown that thinning a sapphire substrate and mounting the sapphire substrate on a heat sink can significantly reduce the heating of the devices. Currently, this thinning procedure is undertaken by backside polishing the sapphire substrate to a thickness of about 100 xcexcm. Continuous wave devices have been achieved by this method, although the thermal resistance of these devices is more than 40 K/W.
Further thinning of the substrates would be desirable to further reduce heating. However, when using conventional techniques like polishing, a further reduction in substrate thickness is difficult to achieve without cracking the substrate. As a result, such thinning procedures have not been satisfactory to reduce heating.
As indicated above, commonly used substrates for group III-V nitride growth have poor thermal conductivities. In contrast, copper has a thermal conductivity of about 4 W/cm-K at room temperature. Copper, however, is not suitable as a substrate for growth of semiconductors, because its melting temperature is lower than the high temperatures required for growth of these materials.
Thus, there is a need for substrates with increased thermal conductivity, and which have the structural integrity of thick substrates.
This invention provides substrates with increased thermal conductivity and methods for forming these substrates.
This invention separately provides substrates having increased thermal conductivity with increased structural integrity.
This invention separately provides semiconductor devices usable as light emitting devices and methods for forming these semiconductor devices.
This invention separately provides semiconductor devices, such as semiconductor laser devices, having decreased sensitivity to self-heating effects.
This invention further provides semiconductor devices grown on substrates where some of the substrate material is replaced with a material having a higher thermal conductivity.
The inventors have discovered that, if there is a region formed of a material having enhanced thermal conductivity connecting the light emitting devices to the external heat sink, then the heat will flow out of the device along the path created by that material. As a result, the temperature of the device will depend less on the dissipated power of the adjacent devices. Thus, having materials with enhanced thermal conductivity between the active region of a device and the external heat sink will lead to lower temperatures during operation and to improved stability of operation. In many instances, such as with multiple monolithically integrated laser diodes, the thermal cross-talk between devices will also be reduced.
The substrates of this invention have a body comprising a material, such as sapphire, that is suitably usable for forming a semiconductor. The substrate body has a top surface and a bottom surface opposite to the top surface. The substrate body has a cavity defined by an inner surface of the substrate body. In various exemplary embodiments, the cavity opens onto at least the bottom surface. In various exemplary embodiments, the cavity contains a material having a greater thermal conductivity than the substrate body. This material is distributed in the cavity so that it is able to transport heat from the top surface of the cavity to an external heat sink placed in the cavity and/or at or below the bottom surface of the substrate. Exemplary embodiments of the semiconductor devices of this invention comprise the substrate described above and at least one semiconductor structure formed over the top surface of the substrate. The semiconductor devices of this invention comprise a structure, such as that described above, and at least one p-contact that contacts the semiconductor structure, where the material in the cavity acts as the n-contact.
In various exemplary embodiments, the method for forming a substrate having increased thermal conductivity includes forming a cavity in a body of a substrate, where the cavity opens on at least a bottom surface of the body, and placing a material having a greater thermal conductivity than the substrate in the cavity. In various exemplary embodiments, the methods according to this invention further include forming at least one semiconductor structure over the top surface of the body.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.